A polarizing film of polyvinyl alcohol based resin with oriented iodine is presently used for many optical display devices such as a television set, a cellular phone, a portable information terminal. In recent years, the polarizing film is increasingly required to be further thinned in the market. Although it is difficult to make a thinned polarizing film having excellent optical properties, the level required for the optical properties has been increasingly raised.
Examples of the manufacturing method of a polarizing film include a manufacturing method comprising a step of stretching a polyvinyl alcohol based resin layer and a resin substrate for stretching in a laminate state, and a step of dyeing. According to the manufacturing method, even a thin polyvinyl alcohol based resin layer can be stretched without problems such as fracture in stretching, due to the support by the resin substrate for stretching.
Examples of the manufacturing method comprising a step of stretching in a laminate state and a step of dyeing include a method of stretching in the air (dry stretching) as described in Japanese Patent Laid-Open No. 51-069644, Japanese Patent Laid-Open No. 2000-338329 and Japanese Patent Laid-Open No. 2001-343521. With respect to the capability of manufacturing a thin polarizing film excellent in optical properties, stretchable with a high stretch ratio, a manufacturing method comprising the step of stretching in a boric acid aqueous solution as described in International Publication No. WO 2010/100917, Japanese Patent Laid-Open No. 2012-073563 and Japanese Patent Laid-Open No. 2012-134117 is advantageous, and, in particular, a manufacturing method (two-stage stretching method) comprising a step of auxiliary stretching in the air prior to stretching in a boric acid aqueous solution as described in Japanese Patent Laid-Open No. 2012-073563 and Japanese Patent Laid-Open No. 2012-134117 is advantageous. In addition, a manufacturing method (excessive dyeing and partial decolorization method) comprising stretching a PVA based resin layer and a resin substrate for stretching in a laminate state, then excessively dyeing the PVA based resin layer, and then partially decolorizing the excessively-dyed PVA based resin layer as described in Japanese Patent Laid-Open No. 2011-2816 is also advantageous.